The present invention relates to a metal vapor discharge lamp using a translucent ceramic arc tube.
This kind of conventional metal vapor discharge lamp is disclosed, for example, in Publication of Japanese Patent Application No. Hei 6-196131 A (conventional lamp 1), No. Hei 7-240184 A (conventional lamp 2), or No. Sho 61-245457 A (conventional lamp 3), etc.
The conventional lamp 1 includes, as shown in FIG. 6, a translucent ceramic arc tube 11 and small tubular portions 12a, 12b provided at both sides of the central main tube portion 13 of the arc tube 11. Inside the small tubular portions 12a, 12b, feeder bodies 14a, 14b are inserted. The feeder bodies 14a, 14b are connected to electrodes 15a, 15b, respectively. The feeder bodies 14a, 14b are made of a hydrogen permeable material 16a, 16b and a halide-resistant material 17a, 17b. The gap between the small tubular portions 12a, 12b and the feeder bodies 14a, 14b is sealed with a glass frit 18a, 18b. 
As the hydrogen permeable material 16a, 16b, niobium, tantalum, or the like, are used, which makes it possible to bring the coefficient of thermal expansion closer to that of alumina that is the material for the small tubular portions 12a, 12b, so as to prevent the occurrence of cracks at the time of sealing. However, niobium etc. is vigorously reacted with a halide that is filled in the main tube portion. Therefore, the halide-resistant material 17a, 17b such as tungsten, molybdenum or a conductive cermet, etc. is used for the member at the portion where the filled material exists during the lamp operation, while the hydrogen permeable portion 16a, 16b made of niobium is completely sealed with the glass frit 18a, 18b. Thus, this configuration inhibits the reaction between the feeder body 14a, 14b and the filled material.
The conventional lamp 2 includes, as shown in FIG. 7, a translucent ceramic arc tube 19, plug bodies 20 and a pair of electrodes 21. The arc tube 19 includes a central bulging portion 22 having a spherical or an elliptic spherical shape, and small tubular portions 23 having a diameter smaller than that of the central bulging portion 22. The small tubular portions 23 extend from both ends of the bulging portion 22, and the small tubular portions 23 and the central bulging portion 22 are formed in one piece. Each plug body 20 is inserted into the small tubular portion 23 and has a conducting means conducting from the inside to outside of the arc tube. The electrodes 21 are provided in the bulging portion 22 and supported by one end of the plug bodies 20, respectively.
In this configuration, an external lead wire 24 that passes through the inside of the plug body 20 conducts from the inside to outside of the arc tube 19. The plug body 20 is bonded to the small tubular portion 23 with glass adhesive 25 made of, for example, a frit glass, which are poured into the gap between the inner surface of the end of the small tubular portions 23 at the opposite side to the electrode 21 and the outer surface of the plug body 20. Furthermore, mercury as a buffer metal, a metal halide as a discharge metal, noble gas such as argon gas, etc. are filled in the arc tube. The filled amount of the metal halide is larger than the amount that evaporates during the lamp operation.
In general, when the temperature of the glass adhesive 25 increases during the lamp operation, the glass adhesive 25 deteriorates due to a chemical reaction with a metal halide. This deterioration causes the occurrence of leaks of the sealed materials from the arc tube. During the lamp operation, in the conventional lamp 2, excess metal halides are condensed in the gap between the inner surface of the small tubular portion 23 and the outer surface of the plug body 20 except for the bonding portion with the glass adhesive 25. This condensed metal halide thermally isolates the glass adhesive 25 from a high temperature gas inside the discharge space. Thus, the deterioration of the glass adhesives 25 due to the chemical reaction with metal halides can be prevented and the occurrence of leaks in the arc tube 19 is prevented.
Furthermore, the conventional lamp 3 has. as shown in FIG. 8, an arc tube including a translucent alumina tube 26, the ends of which are plugged with conductive cermet 27 via a sealing material 28, and dysprosium halide is filled in the arc tube. As a main component of the sealing material 28, an oxide of rare earth metal is used. The conductive cermet 27 is obtained by sintering a mixture of tungsten powder, etc. and aluminum powder, etc., used for the discharge material. Therefore the conductive cermet 27 has the coefficient of thermal expansion that is very close to that of aluminum, so that cracks in the sealed portion can be reduced. Furthermore, since the metal oxide of rare earth metal is used as a main component of the sealing material 28, the reaction between the filled material and the sealing material 28 can be inhibited during the lamp operation.
In the configuration of the conventional lamp as described above, when a metal such as tungsten, molybdenum, or the like, whose coefficient of thermal expansion is different from that of aluminum is used, cracks easily occur in the sealed portion, and leaks easily occur in the arc tube at the step of sealing and during the lamp operation. In order to avoid such disadvantages, it is preferable that the conductive cermet whose coefficient of thermal expansion is close to that of aluminum is used for the halide-resistant portion. However, it is difficult to bond the conductive cermet to niobium as the hydrogen permeable material. Therefore, the reliability in this portion is not obtained and the utilization factor of the feeder body is lowered.
Furthermore, when a metal such as niobium, etc. is used for the feeder body, since the bonding at the interface between niobium and the glass frit is weaker than the bonding at the interface between the glass frit and alumina, i.e. between two oxides, the filled materials gradually leak from the interface between niobium and the glass frit. As a result, the lamp voltage is lowered.
Furthermore, since the coefficient of thermal expansion of niobium is 7.2xc3x9710xe2x88x926, and the coefficient of thermal expansion of alumina is 8.0xc3x9710xe2x88x926, not a little thermal stress occurs at the time of sealing and during the lamp operation. Therefore, in a high power lamp having an electrode rod of a large diameter, the thermal stress is too large to be neglected and cracks occur in the sealed portion. Furthermore, niobium is embrittled due to the reaction with nitrogen at high temperatures. Therefore, in the case of the high power lamp in which the temperature of the ends of the feeder body is easily increased, it is unsuitable to operate the arc tube in a nitrogen atmosphere.
Furthermore, in the configuration in which the ends of the arc tube are sealed with the plug body having an external lead wire that passes through the inside thereof, the bonding between the external lead wire and the plug body is not sufficient and the filled materials leak to the outside from the arc tube along the lead wire, so that the lamp voltage during the lamp operation is significantly lowered.
Furthermore, in the configuration in which the end of the arc tube is sealed with the conductive cermet, since the front surface of the sealing material is close to the discharge space and so has a high temperature, the sealing material is softened, or a sealing material reacts with the filled material. Consequently, the lamp characteristics are significantly deteriorated for a short time.
Furthermore, when the luminous efficiency of the conventional lamps were respectively examined, they were low. For example, the luminous efficiency was about 80 (lm/M) for a high-color-rendering lamp. Although a lamp having a higher luminous efficiency has been desired, improvement of the luminous efficiency has not been considered in the conventional metal vapor discharge lamps.
Furthermore, the luminous flux rise time (time required to obtain the luminous flux of 90% with respect to that of the steady state) at the initial time of the lamp operation was as long as about 13 to 15 minutes. Thus, although the lamp having a shorter luminous flux rise time has been desired, improvement of the luminous flux rise property has not been considered in the conventional metal vapor discharge lamps.
It is an object of the present invention to solve the problems of the prior art. That is, the object of the present invention is to provide a metal vapor discharge lamp having a highly reliable sealing portion realizing the stable lamp characteristics during the lamp operation, and being capable of improving the luminous efficiency and of improving the luminous flux rise property at the initial time of the lamp operation.
According to the present invention, a metal vapor discharge lamp has an arc tube including a discharge portion composed of translucent ceramic in which a discharge metal is filled and a pair of electrodes is disposed; small tubular portions composed of ceramic coupled to both ends of the discharge portion; feeder bodies inserted into the small tubular portions; and a sealing material sealing the gap between the feeder body and the small tubular portion at the end portion opposite to the discharge portion. The surfaces including the respective end faces of the small tubular portions define a discharge space in cooperation with the inner surface of the discharge portion. The feeder bodies are composed of a conductive cermet and the end portions thereof are connected to the respective electrodes. The ends of the conductive cermets on the side opposite to the discharge space extend at least to the ends of the small tubular portions. The temperature of the end face of the sealing material on the discharge space side during the lamp operation is not more than 800xc2x0 C.
According to such a configuration, the bonding strength at the interface between the sealing material and the small tubular portion and conductive cermet in the sealed portion is enhanced and the air-tightness is maintained for a long time. Consequently, when the lamp power is as high as 150 Watt or more, a metal vapor discharge lamp having a highly reliable sealed portion capable of preventing the occurrence of cracks can be realized.
Furthermore, with the configuration in which the temperature of the end face of the sealing material on the discharge space side is limited, the reaction between the sealing material using a glass frit etc. and the filled material can be inhibited. Thus, the metal vapor discharge lamp having the stable lamp characteristics during the lifetime of the lamp can be realized. In addition, since as the feeder body, the conductive cermet is used instead of Nb etc. reacting with nitrogen at high temperatures, nitrogen can be filled in the outer tube in order to reduce the temperature of the sealed portion. Thereby, it is possible to cause a loss of heat at the sealed portion by nitrogen, to lower the temperature of the sealing material and to inhibit the reaction.
As mentioned above, with such a configuration, the stable lamp characteristics can be obtained over the long period of lamp operation. However, the present invention further realizes the metal vapor discharge lamp having a high luminous efficiency and an excellent rise property. More specifically, the present inventor investigated the cause of the deterioration of the luminous efficiency in the conventional metal vapor discharge lamps, and found that the cause was in the heat loss from the discharge space. Also, the present inventor found that the factor to improve the luminous flux rise property was related to the temperature of the filled material. Therefore, the present invention described below is based on such findings.
It is preferable in the above-mentioned configuration that the length L (mm) between the end face of the sealing material on the discharge space side and the discharge space is (3/115)P+355/115 (mm) or more, wherein P denotes the lamp power in watts. Thus, the temperature of the end face of the sealing material on the discharge space side can be 800xc2x0 C. or less. Consequently, the metal vapor discharge lamp in which the lamp characteristics are little changed over the long period of lamp operation can be obtained.
It is preferable that the thermal conductivity of the conductive cermet at 20xc2x0 C. is 0.28 (cal/cmxc2x7secxc2x7deg) or less. Thus, the heat loss caused by heat conduction via the conductive cermet out of the discharge space can be reduced.
It is preferable that the outer diameter r (mm) of the conducting cermet is 4.9xc3x9710xe2x88x923P+0.53 (mm) or less, wherein P denotes the lamp power in watts. Thus, a higher luminous efficiency can be obtained compared to the conventional lamps.
It is preferable that the specific resistance value of the conductive cermet at 20xc2x0 C. is 10.0xc3x9710xe2x88x928 xcexa9m or more and 25.0xc3x9710xe2x88x928 xcexa9m or less. Thus, the temperature of the filled material can be increased promptly at the initial time of the operation of the metal vapor discharge lamp.
Furthermore, it is preferable that the metal vapor discharge lamp includes a heat reserving cover enveloping the small tubular portion. Thus, the reaction between the filled material and the sealing material can be inhibited by adjusting the temperature of the filled material, so that a stable lifetime can be obtained and the desired light color can be obtained.
It is preferable that the arc tube is provided inside the outer tube and nitrogen is filled in the outer tube. Thus, the temperature of the sealed portion can be lowered and the stable lamp characteristics can be obtained during the lifetime of the lamp.